The present invention relates to a battery protective circuit, and more particularly to a configuration for preventing a value of current flowing in a battery from exceeding a guaranteed current when a short or overcharge occurs.
Recently, personal digital assistants such as a portable telephone, a notebook-sized personal computer and a video camera are widely used. These personal digital assistants use a battery for supplying power. A rechargeable secondary battery is used for such a battery.
When current flowing in a battery increases for some reason (a short of an electric circuit or charge at an overvoltage and the like), the battery may generate excessive heat and possibly become degraded or damaged.
Therefore, these tools are conventionally equipped with a battery protective circuit for protecting the battery. An example of the battery protective circuit includes a PTC (Positive Temperature Coefficient) element and a thermal protector. The PTC element or the thermal protector serves as a current and temperature detecting circuit, operating in such a manner that electric resistance thereof increases as a larger current flows in the element and temperature becomes higher, and electric resistance thereof increases rapidly to suppress current when a certain temperature is reached. Further, a thermistor has its resistance value changed as an ambient temperature rises.
The conventional battery protective circuit, however, has a problem as described below. Referring to FIG. 12, the problem of the conventional battery protective circuit will be described.
With respect to FIG. 12, the ordinate and the abscissa respectively represent voltage and current. A represents a charge-guaranteed region in which a battery is rechargeable, and BZ represents a protection region in which a current and temperature detecting circuit such as a PTC element or a thermal protector is functional.
The charge-guaranteed region A represents a relation between current flowing in the battery and voltage across terminals of the battery. The protection region BZ represents a relation between current flowing in the PTC element or the like (and the battery) and voltage across terminals of the PTC element or the like.
The charge-guaranteed region A is a region in which the battery can protect itself, and the protection region BZ is a region in which the current and temperature detecting circuit is functional.
When the value of current flowing in the battery enters the protection region BZ for some reason, the internal resistance of the PTC element or the like increases. As a result, an amount of current flowing in the circuit decreases.
The conventional battery protective circuit has performed its protecting function when heavy current flows, regardless of the charge-guaranteed region A, as shown in FIG. 12.
Therefore, for the current value between the charge-guaranteed region A and the protection region BZ, any safety and reliability of the battery is not assured. Thus, unfortunately, for some types of batteries, the battery cannot protect itself, and in addition, the safety and reliability of a device operated by the battery cannot be assured.
Then, the present invention is made to solve the above mentioned problem, and its object is to provide a battery protective circuit which can ensure the safety and reliability of a rechargeable battery and a device operated by the battery.
According to an aspect, the present invention provides a battery protective circuit for a rechargeable battery, including a current-amount control circuit including a current and temperature detecting circuit provided near the battery, operative to detect a value of current flowing in the battery and an ambient temperature, and to decrease the current value when the current value and the ambient temperature reach a value of a protection region, wherein the minimum current value in the protection region is less than the maximum current value in the charge-guaranteed region in which the battery is rechargeable, and the maximum current value in the protection region is greater than the maximum current value in the charge-guaranteed region.
According to another aspect, the present invention provides a battery protective circuit for a rechargeable battery, including: a current-amount control circuit including a current and temperature detecting circuit provided near the battery, operative to detect a value of current flowing in the battery and an ambient temperature, and to decrease the current value when the current value and the ambient temperature reach a value of a first protection region; and an interconnection layer supplying current to be flown in the battery, including a meltable portion to be melted and cut off when the value of current flowing in the battery reaches a value of a second protection region. The minimum current value in the first protection region is less than the maximum current value in the charge-guaranteed region in which the battery is rechargeable, and the maximum current value in the first protection region is greater than the maximum current value in the charge-guaranteed region. The minimum current value in the second protection region is less than the maximum current value in the first protection region, and the minimum current value in the second protection region is greater than the maximum current value in the charge-guaranteed region. In the interconnection layer, the meltable portion has a relatively small cross sectional area, while a portion other than the meltable portion of the interconnection layer has a relatively large cross sectional area.
Preferably, at least two or more meltable portions of the interconnection layer are arranged.
The aforementioned battery protective circuit can decrease the current value before degradation and damage of the battery, even when the value of current flowing into the battery increases.
As a result, the battery can surely be protected, and the safety and reliability of the battery and the device operated by the battery can be improved.
Further, when the current amount approaches a boundary region of an operating condition of the current and temperature detecting circuit due to overcharge, charge in reverse direction or the like, the interconnection layer is melted and cut off at the time when the current-amount control circuit is not yet damaged, and therefore the current is interrupted.
Therefore, the undesirably high temperature of the battery can be prevented and the current-amount control circuit may not be burdened. As a result, the overall reliability and safety of the device including the battery and the current-amount control circuit can be improved.
According to a further aspect, the present invention provides a battery protective circuit for a rechargeable battery, including: a current-amount control circuit including a current and temperature detecting circuit provided near the battery, operative to detect a value of current flowing in the battery and an ambient temperature, and to decrease the current value when the current value and the ambient temperature reach a value of a first protection region; and an interconnection layer supplying current to be flown in the battery, including a meltable portion to be melted and cut off when the value of current flowing in the battery reaches a value of a second protection region. The minimum current value in the first protection region is less than the maximum current value in the charge-guaranteed region in which the battery is rechargeable, and the maximum current value in the first protection region is greater than the maximum current value in the charge-guaranteed region. The minimum current value in the second protection region is less than the maximum current value in the first protection region, and the minimum current value in the second protection region is greater than the maximum current value in the charge-guaranteed region. In the interconnection layer, the meltable portion has a relatively small cross sectional area, while a portion other than the meltable portion of the interconnection layer has a relatively large cross sectional area. In the interconnection layer, the greater the current value in the second protection, the shorter the time for the meltable portion be melted and cut off.
The aforementioned battery protective circuit can decrease the current value before degradation and damage of the battery, even when the value of current flowing into the battery increases, so that the battery can surely be protected, and in addition, the safety and reliability of the battery and the device operated by the battery can be improved. Furthermore, when the current amount approaches a boundary region of an operating condition of the current and temperature detecting circuit due to overcharge, charge in reverse direction or the like, the interconnection layer is melted and cut off at the time when the current-amount control circuit is not yet damaged, and the current is interrupted. Therefore, the undesirably high temperature of the battery can be prevented and the current-amount control circuit may not be burdened. As a result, the overall reliability and safety of the device including the battery and the current-amount control circuit can be improved. Further, as the current value in the second protection region becomes greater, the time for the meltable portion to be melted and cut off becomes shorter. Therefore, even when heavy current flows, the meltable portion is not melted and cut off, if the time of current flow is short enough. As a result, even when the terminal of the battery causes a momentary short-circuit, the meltable portion does not melt, if the moment is short enough. Therefore, a short circuit over such a short time that does not affect the safety may not result in a failure of the battery.